Electrolytic device



July 28, 1942.

P. ROBINSON 2,290,997 7 ELECTROLYTIC DEVICE Filed Feb. 26, 1931 PRESTON Poe/N50 IN VENTOR.

A TZ'ORNEYS.

Patented 'Jul zs, 1942 UNITED STATES PATENT OFFICE ELECTROLYTIC DEVICE Preston Robinson, North Adams, Mass, assig-nor to Sprague specialtleaco n y, Quincy, Mun, a corporation of Massachusetts Application February 2;, 19:1, Serial No. 51am My invention relates to electrolytic devices such as electrolytic condensers, rectifiers or the like, comprising film-forming electrodes.

In such devices use is made of the film-forming effect exhibited by certain metals, for instance, aluminum, which, when placed in suitable electrolytes, upon application of a proper voltage, form a film oi uni-directional character.

I shall describe my invention as applied to electrolytic condensers and more particularly to those used in filter circuits for rectified alternoting current. However, it should be well understood that my invention is not limited to such application and devices.

In my application Ser. No. 468,466, filed on July 16, 1930, 01 which this application is a com tinuation in part, I have described how, by the addition of certain substances to the electrolyte, I

have succeeded to improve the quality of the film.

and to prevent, to a considerable extent, the electrolyte to attack'the film during periods of idleness. I

The present invention relates to further improvements in the quality of such electrolytic cells, through proper selection of the cathode material.

In the drawing forming part of this specification, the figure is a side elevation partly in section of an electrolytic condenser embodying my invention.

Referring now to the figure, there is shown an electrolytic condenser oi the type used in filter circuits. It comprises a container it which forms the cathode and isprei'erably of non-filming ma.-

terial, and comprises a liquid electrolyte IS, in

which is immersed an aluminum anode I3, shown here as a corrugated tube. The container II is closed by a cover l2 through the center aperture ll, of which projects a threaded extension l5 of the electrode l3, said extension being provided with nuts I8 to form the outside terminals for the anode.

The anode I3 is of the type described in the co-pending applications Ser. No. 416,939, of F. D. sprague and R. U. Clark, filed December 27, 1929, now U. S. Patent 1,958,682 and Ser. No. 441,128, of April 2, 1930, of Max Knab now U. 8. Patent 1,983,468. Such anodes are made out of a single piece of aluminum, and besides other advantages enumerated in the above referred to applications, have the advantage of greatly rethe operation oi the device. A gasket 2! is provided in an upper peripherial recess 22 of the cover i8, which provides for an air-tight seal between the cover and the container. As a rule, additional sealing means are provided-not shownto prevent leakage between the cover It and the stern of the anode l3 projecting therethrough.

As is well known, the is formed on the anode by subjecting the condenser to gradually increasinr, electrical potentials, with properly regulated current flow; the film, which consists Of partially hydrated isn't oxide, thereby gradually builds up on the surface of the aluminum.

In use of the condensers, it is relied upon that the film once properly formed will remain u...- changed and retainits high resistance at voltages oi proper polarity as long as their values do not exceed the maximum forming voltage.

This is true to a great extent when the can denser is in continuous operation under the prop or voltage conditions. However, when after a period of idleness thecondenser is again placed in operation, a current surge takes place due to two causes. One cause is the loss of charge of the condenser, due to leakage; this is a transient current of very short duration, not specific to electrolytic condensers, and can be disregarded as a rule. The second cause for a current 'surge is specific with electrolytic condensers and is I accompanied by a lowering of the breakdown voltage. The reasons therefor will be explained hereafter.

when the condenser is in operation, the film does not dissolve in theelectrolyte; however,

when the condenser is idle, as pointed out in my above-referred to application, a slow dissolution of the film in the electrolyte takes place,

which is partly responsible for the current surge.

The dissolution of the film and thus of the surge is dependent on the duration of the peri- I ods of idleness of the condenser. These, when the condenser is used in filter circuits of radio ducing the leakage current of condensers in receiving sets, are long compared with the periods of operation;

When the condenser, after a prolonged idleness is placed again in operation, the film, having been partly dissolved, offers a reduced resistance to the current flow. While this current flow gradually rebuilds the dissolved film, in the meantime the excess current might easily damage other devices inserted in the condenser circult.

At the same time the reduced resistance or the filmmaused by its partial dissolution, reduces the breakdown voltage, and even after the film has been gradually rebuilt, the original breakdown voltage will not be obtained, as complete repair of the film woulclrequire a slow and gradual raising of the voltage, similar to that used in the initial forming process. In fact, after long periods of idleness, the initial quality of the film could be only restored'by carefully repeated formation. This, however, is unfeasible in practice, and the above phenomenon permanently deteriorates the quality of the film, reducing the breakdown voltage and increasing the leakage current of the condenser.

In my application above referred to, I have described how, by saturating the electrolyte with an excess of a precipitate of the same constituency as'the film, dissolution of the film during the periods of idleness can be prevented. In case of the aluminum electrodes, this precipitate is aluminum oxide or aluminum hydroxide. As a preferred method, I have also suggested to use a anode widely varies, and that the material of hydrous gelatinous precipitate of the same constituency as the film, but having a higher rate of solubility than the film.

The advantage of my above invention manifests itself in the formation of a better quality film, which results in a lower initial leakage current and higher initial breakdown voltage of the condenser, and in a reduction of the current surge after periods of idleness, and in a corresponding reduction of the ill-effects incidental to such surge.

I have, however, found that besidesv the above described dissolution of the film, there is another contributing reason for the large initial leakage currents, shown by present day condensers after periods of idleness.

As stated previously, when an electrolytic condenser is used for the filtering out of A. C. ripples from a rectified current, the cathode is preferably selected to be of a non-filming metal. The reason, therefore, is that as in practice the A. C'. ripples to be filtered out may cause a film to be formed on the cathode, which reduces the capacity of the condenser and increases its power factor. Various non-filming metals have been suggested as cathode material, for instance, iron and copper.

I have found, however, that metals, as iron and copper, when used as cathode material, will contribute, for reasons hereafter described, to a marked degree in increasing the leakage current after idleness. I

When the condenser is idle, a portion of such cathode metals is attacked by the electrolyte and forms salts which are carried in the electrolyte to the anode. These salts are capable of being reduced by aluminum, and since the oxide film on the aluminum anode is not entirely impervious to the passage of the electrolyte, a portion of these salts comes in contact with the aluminum anode and is reduced by the aluminum. This results in the deposition of cathode metal upon the anode, and when the condenser is again placed in operation, this deposit will cause the leakage current to be increased and the breakdown voltage to be reduced similarly as has been caused by the dissolving action of the electrolyte previously described. The leakage current, due to this reason, also decreases gradually, as the foreign metals are oxidized and the oxide passes into the electrolyte.

I have now found that for different metals the character of such metallic deposits on the found that the amount of the cathode, depending on its behavior as a deposit -on the anode, influences the leakage current and breakdown voltage of the condenser after a period of idleness.

For instance, iron and copper form a closely adherent deposit covering the surface of the anode and cause -a relatively large leakage current after idleness of the condenser, while other metals, notably nickel and cobalt, under similar conditions, are reduced to drops off the anode and sinks to the bottom of the vessel, and, therefore, do not contribute, or only to a small extent, to the leakage current, after idleness of the condenser.

I have found, for instance, thatafter a given period of idleness, all other circumstances being equal, the transient leakage current flowing through a copper-cathode condenser required twice as much time to die down than through a nickel-cathode condenser.

Nickel and cobalt have, however, further advantages as cathode material over iron and cop-' per, which will be hereafter explained.

The various metals generally referred to as non-filming exhibit a marked film-forming effect which is more or less pronounced, depending on the electrolyte used in the condenser. When condensers are used for usual A. C. ripples of the order of 30 to volts from a rectified current, such film-formation takes place, which, however, due to. a secondary eflect, namely, the liberation of hydrogen at the cathode, and the consequent reduction of the oxide film, is at least partly obscured. I have hydrogen liberated, the amount of oxide film reduced by the hydrogen and thus the actual film-formation on the cathode, depends on the amount of leakage-current through the condenser. The smaller the leakage current, the less hydrogen is liberated and the more pronounced the film-formation.

For reasons well known in the art, it is on the other hand advisable to keep the leakage current as low as possible. By using materials of high purity, using the improved form of electrodes as described in the above referred to applications of F. D. Sprague and R. U. Clark, and of Max' Knab, improved methods of cleaning the electrodes, as described in the corresponding application of Preston Robinson and J. L. Collins, Ser.

No. 526,118, filed Marchv 28, 1931, now U. S.-

' Patent 2,067,703 and improved electrolytes and process of film-formation as described in my above referred to copending application, condensers having a very small leakage current can be obtained.

While in case of condensers having a considerable leakage current, the amount of liberated hydrogen might be suflicient to prevent deleterious film-forming on an iron or copper cathode, I have found that when the leakage current is very trolyte of the order of .05 to .2 milliamp. per microfarad for 450 volts, the hydrogen liberated is insufficient to prevent altogether film-formation on an iron or copper cathode. The'result is that in use the power factor of such condensers increases and their capacity decreases, which, for well-known reasons, is highly objectionable. On the other hand, when nickel or cobalt is used as cathode material, ,no objectionable film-formation results even" with leakage current of the above or of smaller values.

For instance, in comparing condensers with a fine powder which the filtering out of thelow, for instance, in case of the above eleccopper cathodes and nickel cathodes, which condensers are otherwise in every respect identical and have the same initial power factor and capacity, I have found that after a period of two weeks the power factor of the copper cathode condensers increases to above its initial value and the capacity decreases. On the other hand, with nickel cathodes, no change in the power factor and capacity was found even after a period of several months use.

The use of nickel and cobalt as cathode material, besides having the advantage of not causing an adherent metal deposit on the anode, and

of not showing film-formation on the cathode even in condensers of high leakage resistance, has the further advantage that those metals, having a higher solution potential than themetals usually found as impurities in the electrolyteas iron, copper, lead or tin-and these impurities are plated out on the nickel.

As nickel and cobalt are rather expensive metals, I have repeated my tests in employing a less expensive base metal, for instance, copper, brass or iron, and plated thereon an impervious nickel or cobalt coating. Thereby I obtained results which are just as good as when using massive nickel or cobalt cathodes. While I prefer to make the nickel plating so as to completely and imperviously cover the base metal on its surface contacting with the electrolyte, I have found that small discontinuities of the plating are not fatal, because as long as the exposed surface of the base metal is small, the base metal will plate out on the nickel or cobalt instead of going into the electrolyte.

While I have described my invention in connection with specific devices and in a specific application, I do not wish to be limited to such devices or applications, but desire the appended claims to be construed as broad and permissible in view of the prior art.

What I believe to be new and desire to secure by Letters Patent is:

i. In an electrolytic condenser having a filmed aluminum anode, an electrolyte comprising weak acid and a salt of a weak acid and a precipitate of aluminum oxide which is more soluble in the electrolyte than the film and a cathode of one of the metals nickel and cobalt, the salts of which when reduced by the aluminum do not form a coherent deposit on the anode.

2. An electrolytic condenser of rectified current having A. C. ripples of the order of to volts and a D. C. component of the order of 450 volts, comprising a container,. an electrolyte and a filmed electrode provided with an aluminum oxide film in-said container, said container consisting of a base of a. ductile and substantially non-filming metal and having a coating of one of the metals nickel and cobalt on all its portions accessible to the electrolyte, said electrolyte comprising a weak acid and a salt of a weak acid, and an excess of a precipitate of the same constituency as that of the film of the electrode, said container forming the cathode of the condenser I through which pass the A. C. current ripples.

PRESTON ROBINSON. 

